The present invention relates to a process for making a lube base stock from materials having lower molecular weights. Included in this invention is a process for making predominately bright stock lube base stock.
Lubricant oils of high viscosity and high oxidation stability are desirable. Such materials have been prepared, for example, by hydrocracking, hydrodewaxing and hydrofinishing various petroleum feedstocks and by polymerizing normal alpha olefins such as 1-decene. The former route has the advantage of moderate costs, but the oxidation stability is not exceptional. As attempts are made to improve the oxidation stability by increasing the severity of the hydroprocessing steps, the yield of lube declines, as does its viscosity. The latter route gives an exceptionally stable product, but suffers the disadvantage of high cost.
It would be desirable to provide a moderate cost process that generates high viscosity and highly stable products. The present invention provides such a process.
U.S. Pat. No. 6,025,533 to Vora, et al. (xe2x80x9cOligomer Production with Catalytic Distillationxe2x80x9d) teaches production of heavy oligomers (C7+oligomers) from C4 paraffins and olefins by a combination of dehydrogenation and oligomerization. The process has at least one catalyst bed in the top of a distillation column for separating the oligomerization effluent of the dehydrogenation and oligomerization combination.
U.S. Pat. No. 5,276,229 to Buchanan, et al. (xe2x80x9cHigh VI Synthetic Lubricants From Thermally Cracked Slack Waxxe2x80x9d) teaches oligomerizing alpha-olefins produced from thermal cracked slack wax.
U.S. Pat. No. 5,015,361 to Anthes, et al. (xe2x80x9cCatalytic Dewaxing Process Employing Surface Acidity Deactivated Zeolite Catalystsxe2x80x9d) teaches oligomerization of propylene in two stages using ZSM-23 and ZSM-5 to form a low pour point, high cloud point product, followed by dewaxing.
U.S. Pat. No. 4,855,524 to Harandi, et al. (xe2x80x9cProcess For Combining The Operation of Oligomerization Reactors Containing a Zeolite Oligomerization Catalystxe2x80x9d) teaches combining the operation of a primary reactor that oligomerizes a C3-7 feed to gasoline range hydrocarbons and a high pressure secondary reactor that oligomerizes the effluent of the first reactor to make distillate or lubes.
U.S. Pat. No. 4,678,645 to Chang, et al. (xe2x80x9cConversion of LPG Hydrocarbons to Distillate Fuels or Lubes Using Integration of LPG Dehydrogenation and MOGDLxe2x80x9d) teaches converting C4xe2x88x92 paraffins to higher hydrocarbons by the combination of catalytic or thermal dehydrogenation of a paraffinic feedstock to produce olefins and conversion of olefins to gasoline and distillate boiling range materials in a low pressure oligomerization catalytic reactor and a high pressure oligomerization catalytic reactor.
A variety of patents disclose catalysts useful for oligomerization.
U.S. Pat. No. 5,453,556 to Chang et al. Oligomerization Process For Producing Synthetic Lubricantsxe2x80x9d) teaches an oligomerization process using a catalyst having an acidic solid with a Group IVB metal oxide modified with an oxyanion of a Group VIB metal.
U.S. Pat. No. 5,270,273 to Pelrine et al. (xe2x80x9cOlefin Oligomerization Catalystxe2x80x9d) teaches an olefin oligomerization catalyst having a supported, reduced Group VIB metal oxide on an inorganic support, such as MCM-41.
U.S. Pat. No. 5,243,112 to Chester, et al. (xe2x80x9cLubricant Range Hydrocarbons From Light Olefinsxe2x80x9d) teaches oligomerizing an olefinic feedstock over a medium pore zeolite catalyst (HZSM-22).
U.S. Pat. No. 5,171,909 to Sanderson, et al. (xe2x80x9cSynthetic Lubricant Base Stocks From Long-Chain Vinylidene Olefins and Long-Chain Alpha- and/or Internal-Olefinsxe2x80x9d) teaches oligomerization of long-chain olefins using certain acidic montmorillonite clay catalysts.
U.S. Pat. No. 5,146,022 to Buchanan et al (xe2x80x9cHigh VI Synthetic Lubricants From Cracked Slack Waxxe2x80x9d) teaches oligomerizing with a Lewis acid catalyst a mixture of C5-C18 or C6-C16 alpha-olefins produced from thermal cracking of slack wax.
U.S. Pat. No. 5,080,878 to Bowes, et al. (xe2x80x9cModified Crystalline Aluminosilicate Zeolite Catalyst and Its Use in the Production of Lubes of High Viscosity Indexxe2x80x9d) teaches oligomerization with a modified zeolite (ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38, or ZSM-48).
U.S. Pat. No. 4,962,249 to Chen, et al. (xe2x80x9cHigh VI Lubricants From Lower Alkene Oligomersxe2x80x9d) teaches oligomerization of lower olefins with a reduced valence state Group VIB metal oxide on porous support. In one embodiment, a feedstock of lower olefins is contacted with surface deactivated, acidic, medium pore, shape selective metallosilicate catalyst under oligomerization conditions, then reacting the mixture with ethylene in contact with an olefin metathesis catalyst under metathesis conditions, then oligomerizing the metathesis product in contact with a reduced valence state Group VIB metal catalyst on porous support.
U.S. Pat. No. 4,542,251 to Miller (xe2x80x9cOligomerization of Liquid Olefin Over a Nickel-Containing Silicaceous Crystalline Molecular Sievexe2x80x9d) teaches oligomerization in the liquid phase using nickel-containing silicaceous crystalline molecular sieve catalysts to produce lube base stock.
U.S. Pat. No. 4,417,088 to Miller (xe2x80x9cOligomerization of Liquid Olefinsxe2x80x9d) teaches oligomerization of liquid olefins using intermediate pore size molecular sieves to produce lube base stock.
EP 791,643 A1 (xe2x80x9cLubricating Oilsxe2x80x9d) teaches a process for the production of lubricating oils having a viscosity index of at least 120 and a pour point of xe2x88x9245 C. or less by oligomerizing a feedstock comprising one or more C5-18 1-olefins in the presence of an oligomerization catalyst comprising an ionic liquid.
In conventional hydrodewaxing, the pour point is lowered by selectively cracking the longer chain wax molecules, mostly normal and slightly branched paraffins. A disadvantage associated with catalytic dewaxing is that the wax is degraded to lower molecular weight materials. For example, waxy paraffins may be cracked down to butane, propane, ethane and methane and so may branched paraffins which do not contribute to the waxy nature of the oil. It is desirable to limit the degree of cracking which takes place during a catalytic dewaxing process, because these lighter products are generally of lower value than the higher molecular weight materials, and because the viscosity index and oxidation stability of the resulting oil is degraded by the loss of paraffins.
A major breakthrough came with the discovery of new dewaxing catalysts, which were found to isomerize rather than crack the wax molecules. Isomerization alters the molecular structure of wax molecules, and generally decreases the pour point of a molecule without significantly changing its boiling point. In contrast to wax cracking, isomerized molecules are retained in the lubricating oil base stock, increasing yield of lubricating oil base stock without reducing viscosity index or oxidation stability significantly.
U.S. Pat. No. 5,135,638 to Miller (xe2x80x9cWax Isomerization Using Catalyst of Specific Pore Geometryxe2x80x9d) discloses a process for producing lube oil from a feedstock having greater than 50% wax. The feedstock is isomerized over a catalyst comprising a molecular sieve (e.g., SAPO-11, SAPO-31, SAPO-41, ZSM-22, ZSM-23, and ZSM-35) and at least one Group VIII metal at a pressure of from about 15 psig to about 2000 psig.
U.S. Pat. No. 5,246,566 to Miller (xe2x80x9cWax Isomerization Using Catalyst of Specific Pore Geometryxe2x80x9d) discloses a process similar to that of U.S. Pat. No. 5,135,638, but where the waxy feed has a pour point of above about 0 C. and contains greater than about 70% paraffinic carbon.
U.S. Pat. No. 5,282,958 to Santilli, et al. (xe2x80x9cUse of Modified 5-7 xc3x85 Molecular Sieves For Isomerization of Hydrocarbonsxe2x80x9d) discloses isomerizing a feed including straight chain and slightly branched chain paraffins having 10 or more carbons using an intermediate pore size molecular sieve (e.g., SAPO-11, SAPO-31, SAPO-41, ZSM-22, ZSM-23, and ZSM-35). Feeds that may be processed by this method include waxy feeds, which contain greater than about 50% wax.
U.S. Pat. No. 5,082,986 to Miller (xe2x80x9cProcess for Producing Lube Oil From Olefins By Isomerization Over a Silicoaluminophosphate Catalystxe2x80x9d) discloses a process for making a C20+ lube oil from olefins or reducing the pour point of a lube oil comprising isomerizing the olefins over a catalyst an intermediate pore size silicoaluminophosphate molecular sieve and at least one Group VIII metal.
Large pore zeolites represent another class of catalysts that have been taught for wax isomerization.
EP 464,546 to Degnan et al. (xe2x80x9cProduction of high viscosity index lubricantsxe2x80x9d) teaches producing a high viscosity index lubricant from a petroleum wax feed having a paraffin content of at least 40 weight percent. The catalyst is a low acidity zeolite isomerization catalyst having an alpha value of below 20. Zeolite Beta, which contains boron as a framework component of the zeolite, is taught as being preferred.
WO 96/26,993 to Apelian et al. (xe2x80x9cWax Hydroisomerization Processxe2x80x9d) teaches for producing a high viscosity index lubricant catalytically dewaxing waxy paraffins by isomerization in the presence of hydrogen and a low acidity large pore zeolite isomerization catalyst having a ratio of SiO2/Al2O3, as synthesized, of at least 200:1.
WO 96/13,563 to Apelian et al. (xe2x80x9cWax Hydroisomerization Processxe2x80x9d) teaches an isomerization process for producing a high viscosity index lubricant using a low acidity large pore molecular sieve having a crystal size of less than 0.1 micron, an alpha value of not more than 30 and containing a noble metal hydrogenation component.
EP 225,053 to Garwood et al. (xe2x80x9cLubricant production processxe2x80x9d) teaches isomerization dewaxing using a large pore, high silica zeolite dewaxing catalyst, followed by a subsequent dewaxing step which selectively removes the more waxy n-paraffin components. The selective dewaxing step may be either a solvent or a catalyst dewaxing, preferably using a highly shape selective zeolite such as ZSM-22 or ZSM-23.
EP 659,478 to Perego et al. (xe2x80x9cProcess for preparing amorphous, catalytically active silico-sluminasxe2x80x9d) teaches a process for producing a high VI lubricant from a waxy hydrocarbon feed by isomerization in the presence of hydrogen and a low acidity large pore molecular sieve.
Non-zeolitic catalysts are also taught for wax isomerization.
U.S. Pat. No 5,049,536 to Belussi, et al. (xe2x80x9cCatalytically Active Silica and Alumina Gel and Process For Preparing Itxe2x80x9d) catalysts are described based on silica and alumina gel and their use in isomerization processes.
EP 582,347 to Perego et al. (xe2x80x9cCatalyst for the hydroisomerization of long-chain N-paraffins and process for preparing itxe2x80x9d) teaches a bifunctional catalyst for hydroisomerization. That catalyst has at least one Group VIIIA noble metal on a calcined amorphous silica and alumina gel.
U.S. Pat. No. 5,723,716 to Brandes, et al. (xe2x80x9cMethod For Upgrading Waxy Feeds Using a Catalyst Comprising Mixed Powdered Dewaxing Catalyst and Powdered Isomerization Catalyst Formed Into A Discrete Particle (LA W082)xe2x80x9d) teaches combinations of zeolitic and non-zeolitic catalyst components.
U.S. Pat. No. 6,008,164 to Aldrich et al. (xe2x80x9cLubricant Base Oil Having Improved Oxidative Stabilityxe2x80x9d) teaches a method of producing a lube base stock by separating, into a plurality of fractions based on molecular shape, a hydroisomerized hydrocarbon wax, and collecting the fractions that have a preselected oxidative stability.
U.S. Pat. Nos. 4,417,088; 4,542,251; 4,678,645; 4,855,524; 4,962,249; 5,015,361; 5,049,536; 5,080,878; 5,082,986; 5,135,638; 5,146,022; 5,171,909; 5,243,112; 5,246,566; 5,270,273; 5,276,229; 5,282,958; 5,453,556; 5,723,716; 6,008,164; and 6,025,533 are hereby incorporated by reference for all purposes.
The present invention provides a process for forming hydrocarbons in the lube base stock range from a lower molecular weight feedstock. A highly paraffinic feedstock with boiling points greater than 180 F., preferably greater than 258 F., more preferably within the range of from 258 to 1100xc2x0 F., most preferably within the range of from 258 to 650 F., and preferably including greater than 75% by weight paraffins, is obtained. Preferably, the highly paraffinic feedstock is purified to remove oxygenates and other impurities, for example, by hydrotreatment or by adsorption with an acid clay. Preferably, the highly paraffinic feedstock is dehydrated and decarboxylated to convert any alcohols or acids which may be present to olefins.
The paraffinic feedstock is dehydrogenated in a dehydrogenation zone to produce an olefinic feedstock that preferably includes less than 50% olefins by weight, more preferably between 10% and 50% olefins by weight, with the balance being predominantly paraffins. When paraffinic feedstocks are dehydrogenated, the dehydrogenation conditions can form undesired diolefins. Preferably, the olefinic feedstock is selectively hydrogenated to saturate at least a portion of any diolefins which may be present while retaining the monoolefins. Conditions for selective hydrogenation of diolefins in the presence of monoolefins are well known to those of skill in the art.
The olefinic feedstock is contacted with an oligomerization catalyst in an oligomerization zone to produce a product having a higher number average molecular weight than the olefinic feedstock. Preferably, the product from the oligomerization zone has a number average molecular weight at least 10% higher than the olefinic feedstock, more preferably at least 20% higher than the olefinic feedstock. Preferably, the oligomerization catalyst includes an inorganic oxide support, more preferably a Group VIII metal on an inorganic oxide support, most preferably a Group VIII metal on a zeolitic support. In one embodiment, the oligomerization catalyst is nickel on ZSM-5. In an alternative embodiment, the oligomerization catalyst comprises an ionic liquid, preferably an acidic ionic liquid.
In one embodiment, the oligomerization zone is located within a catalytic distillation unit used to both produce the product and separate the product into a light byproduct fraction and a heavy product fraction. In that embodiment, the olefinic feedstock can also be contacted with an oligomerization catalyst in a fixed bed prior to the catalytic distillation unit. Preferably, at least a portion of the light byproduct fraction is recycled either to the catalytic distillation unit or to the fixed bed or to both the catalytic distillation unit and the fixed bed.
The product from the oligomerization zone is separated into a light byproduct fraction and a heavy product fraction, where the heavy product fraction comprises lube base stock. The production of hydrocarbons in the lube base stock range can maximized by recycling substantially all of the light byproduct fraction, either to the dehydrogenation zone, to a cracking zone, or to a catalytic distillation zone. Preferably, at least a portion of the light byproduct fraction is recycled to the dehydrogenation zone. The paraffinic feedstock, olefinic feedstock, and/or final product can be subjected to skeletal isomerization to control the pour and cloud point of the final product. Skeletal isomerization can be induced at any of a number of points of the process, including (1) on the highly paraffinic feedstock, (2) during dehydrogenation, (3) on the olefinic feedstock (4) in the oligomerization zone, (5) on the product from the oligomerization zone, and/or (6) on the heavy product fraction. Preferably, skeletal isomerization is induced prior to the oligomerization zone. The product from the oligomerization zone or the heavy product is advantageously subjected to hydrofinishing conditions to reduce the olefin content.
The hydrocarbons in the lube base stock range produced by the process include predominantly paraffins and are free of aromatics. Since paraffins are less susceptible to oxidation than aromatics, the product has higher oxidation stability than aromatic-containing compositions.
Preferably, the heavy products fraction has a viscosity of greater than 2 cSt at 100xc2x0 C., a viscosity index of at least 80 and a pour point of less than xe2x88x9210xc2x0 C. More preferably, the viscosity index is at least 120 and a pour point of less than xe2x88x9220xc2x0 C. More preferably, heavy products fraction is separated into at least one of the following fractions:
a) a light lube base stock fraction having a viscosity of from 2 to 7 cSt at 100xc2x0 C.;
b) a heavy lube base stock fraction having a viscosity of from 6 to 20 cSt at 100xc2x0 C.; and
c) a bright stock fraction having a viscosity of greater than 180 cSt at 40xc2x0 C.
In one embodiment, the highly paraffinic feedstock is derived, in whole or in part, from Fischer-Tropsch synthesis. Preferably, the product of the Fischer-Tropsch synthesis is separated into a light gas fraction, a middle fraction (which is heavier than the light gas fraction and which forms at least part of (preferably substantially all of) the highly paraffinic feedstock), and a wax fraction (which is heavier than the middle fraction).
In one embodiment, the wax fraction can be thermally cracked to provide olefins, rather than using paraffin dehydrogenation to provide olefins, and the resulting olefinic feedstock sent to the oligomerization zone. In another embodiment, a portion of the middle fraction and the entire wax fraction is thermally cracked to form an olefinic feedstock, and at least a portion of this feedstock is sent to the oligomerization zone.